Process for producing Ce-Mn coactivated fluoroapatite phosphors as the yellow emitting component for high efficacy lamp blends

ABSTRACT

Yellow emitting fluorescent lamp phosphor compositions comprise cerium and manganese activated alkali-earth fluoroapatite compositions containing sodium having the approximate formula 
     
         (Ca.sub.1-s Sr.sub.s).sub.t Ce.sub.v Na.sub.w Mn.sub.x (PO.sub.4).sub.y 
    
      F z   
     wherein 
     s is from 0 to 1 
     t is from about 3.43 to about 4.01 
     v is about 0.3 
     w is about 0.3 
     x is from about 0.3 to about 0.4 
     y is about 3.00 
     z is from about 1.0 to about 4.0. 
     An improved two step firing process for producing the disclosed phosphors is described and an improved fluorescent lamp employing the disclosed phosphors is also described.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation of co-pending application Ser. No. 467,674 filedon Feb. 18, 1983 which is a continuation in part of Ser. No. 373,384filed Apr. 30, 1982, both now abandoned. A co-pending patent applicationSer. No. 373,330 filed Apr. 30, 1982, entitled "YELLOW EMITTINGPHOSPHORS, PROCESS FOR PRODUCING SAME, AND A FLUORESCENT LAMP CONTAININGSAME" by Romano G. Pappalardo AND John Walsh, and assigned to GTELaboratories Incorporated, assignee of this application, concernsrelated subject matter of this application.

FIELD OF THE INVENTION

This invention relates to alkaline earth fluoroapatite luminescentmaterials. More particularly, it relates to calcium-strontiumfluoroapatite phosphors activated with cerium and manganese and tofluorescent lamps incorporating these phosphor materials.

BACKGROUND OF THE INVENTION

British Pat. No. 792,598 to Ranby and corresponding J. ElectrochemicalSoc., Vol. 104, No. 10 (October 1957)pp. 612-615, article entitled"Cerium-Activated Halophosphate Phosphors" by S. T. Henderson and P. W.Ranby disclose a luminescent material comprising a halophosphate ofcalcium and/or strontium, having an apatite structure activated withcerium or with cerium and manganese, containing sodium.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, there isprovided an improved luminescent composition comprising a cerium andmanganese activated calcium-strontium fluoroapatite compositioncontaining sodium having the approximate formula

    (Ca.sub.1-s Sr.sub.s).sub.t Ce.sub.v Na.sub.w Mn.sub.x (PO.sub.4).sub.y F.sub.z

wherein

s is from 0 to 1

t is from 3.43 to about 4.01

v is about 0.3

w is about 0.3

x is from about 0.3 to about 0.4

y is about 3.0

z is from about 1.0 to about 4.0.

In another aspect of the invention, a fluorescent lamp comprising alight-transmitting envelope has electrodes, an inert ionizable gas and acharge of mercury therein and a coating of phosphor on the insidesurface of the envelope. The phosphor comprises a cerium and manganeseactivated calcium-strontium fluoroapatite composition containing sodiumhaving the approximate formula

    (Ca.sub.1-s Sr.sub.s).sub.t Ce.sub.v Na.sub.w Mn.sub.x (PO.sub.4).sub.y F.sub.z

wherein

s is from 0 to 1

t is from 3.43 to about 4.01

v is about 0.3

w is about 0.3

x is from about 0.3 to about 0.4

y is about 3.0

z is from about 1.0 to about 4.0.

In still another aspect of the invention, a process for producing ceriumand manganese activated calcium-strontium fluoroapatite luminescentmaterial composition containing sodium comprises comminuting thematerial composition to form a thoroughly mixed powder blend. The powderblend is then fired for about 1 to about 1.5 hours at about 900° C. toabout 1050° C. in about 1.0 l/minute flow of N₂ containing 5% H₂ and asmall amount of water vapor. The fired blend is then cooled to about700° C. in about 2 hours and further cooled to about 25° C. The cooledfired blend is then milled into a fine powder.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a view of a fluorescent lamp, partially in section,diagrammatically illustrating an embodiment of the invention;

FIGS. 2 through 6 are the emission spectrum for a number of the phosphorcompositions in accordance with the present invention and the emissionspectrum of a type 4381 standard phosphor compared with each; and

FIGS. 7 and 8 are the excitation spectrum for two of the phosphorcompositions in accordance with the present invention and the excitationspectrum of a type 4381 standard phosphor compared with each.

For a better understanding of the present invention; together with otherand further objects, advantages and capabilities thereof, reference ismade to the following disclosure and appended claims in connection withthe above-described drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings with greater particularity, there is shownin FIG. 1 a fluorescent lamp 10. Lamp 10 comprises an elongated sealedglass envelope 12 of circular cross section. It has the usual electrodes14 and 16 at each end supported by lead-in wires 18, 20 and 22, 24,respectively, which extend through glass presses 26, 28 in mount stems30, 32 to the contacts in bases 34, 36 affixed to the ends of the lamp10.

Envelope 12 is filled with an inert gas such as argon or a mixture ofargon and neon at a low pressure, for example, two torr, and a smallquantity of mercury, at least enough to provide a low vapor pressure ofabout six microns during operation.

The interior of envelope 12 is coated with a layer of phosphor 38 of thepresent invention.

A phosphor coating suspension was prepared by dispersing the phosphorparticles in a water base system employing polyethylene oxide as thebinder with water as the solvent.

The phosphor suspension was applied in the usual manner of causing thesuspension to flow down the inner surface of envelope 12 and allowingthe water to evaporate, leaving the binder and phosphor particlesadhered to the envelope 12 wall. The phosphor coated envelope 12 wasthen heated in an oven to volatilize the organic components, thephosphor layer 38 remaining on the envelope 12 wall.

Envelope 12 is processed into a fluorescent lamp by conventional lampmanufacturing techniques.

Higher values of the efficacy (lumens per watt) have been obtained inlow pressure fluorescent lamps by using a combination of two phosphors,namely, a narrow band, blue emitting phosphor, specifically, a europiumactivated strontium chloroapatite phosphor and a narrow band, yellowemitting phosphor based on an antimony and manganese activated calciumfluoroapatite phosphor. It is the purpose of the present invention tomake a narrow band, yellow emitting phosphor that may be used instead ofan antimony and manganese activated calcium fluoroapatite Sylvania type4381 phosphor (henceforth designated as type 4381) in high efficacy, twocomponent lamp blends. These phosphors are based on the alkali-earthfluoroapatites (with the alkali-earth being Ca, Sr or a combination ofthe two) and contain as activators Ce and Mn. In these phosphors, Mnprovides the needed narrow band yellow emission, while Ce is used tosensitize the Mn emission. A co-pending patent application, Ser. No.373,330, covers similar materials formulated with excess fluoride. Thematerials of this invention are formulated with both fluoride excess andcompensation of the trivalent cation Ce³⁺ by sodium. A formulationapproaching optimum light output, for the use in high efficacy lamps,was found initially to be:

    Me.sub.4.0005 Ce.sub.0.3 Na.sub.0.3 Mn.sub.0.3 (PO.sub.4).sub.3 F.sub.2

(where Me equals Ca, Sr and combinations thereof).

The formulations used in the preparation of the phosphors of the presentinvention are different from those expected to yield a stoichiometricapatite. In particular, these formulations are cation deficient andrequire an excess of halide relative to a stoichiometric apatiteformulation. Nevertheless, X-ray powder diffractometry shows that theseformulations do yield, after firing, apatite as the only detectablyphase. It is believed, therefore, that the stoichiometric imbalance iscorrected during the phosphor-forming heat treatment either throughvaporization losses or through the formation of a second glassy phase.Moreover, it has been found that preparation of the most efficientphosphors requires a formulation providing both a fluoride excess andcompensation of the trivalent Ce³⁺ by a monovalent ion such as Na⁺.Fluoride formulations ranged from 1 to 4 moles of F per 3.0 moles of(PO₄). The optimum fluoride formulation was found to be 2 moles F per3.0 moles of (PO₄).

The phosphors of the present invention were prepared by milling togetherthe luminescent grade reagents that are identified as to type andquantity in Examples 1 through 6 and Examples 7 through 10. After thereagents were thoroughly mixed, the resulting powder blend was placed inan alumina tray that was covered and heated to 1025° C. in a stream of95/5 (% vol) N₂ /H₂. Before passing into the furnace, the N₂ --H₂ gasmixture was bubbled through a water bath maintained at a temperature of≃21° C. The presence of a small amount of water vapor in the gas streamwas found to be necessary to the consistent preparation of phosphorswith white body color. The tray containing the phosphor blend was heldat 1025° C. for about 1 hour and was then allowed to furnace-cool toabout 700° C. in 2 hours before moving it into the furnace vestibulearea where it cooled to room temperature. After cooling, the phosphor isin the form of a soft cake that is easily milled into a fine powder forsubsequent use.

EXAMPLE 1 Formulation Ca₄.0005 Ce₀.3 Na₀.3 Mn₀.3 (PO₄)₃ F₂

Reagent Blend

CaHPO₄ : 8.16 grams

CaCO₃ : 0.01 grams

CaF₂ : 1.56 grams

CeO₂ : 1.03 grams

MnCO₃ : 0.69 grams

Na₂ CO₃ : 0.32

EXAMPLE 2 Formulation Ca₃.955 Ce₀.3 Na₀.3 Mn₀.35 (PO₄)₃ F₂

Reagent Blend

CaHPO₄ : 8.15 grams

CaCO₃ : 0.81 grams

CaF₂ : 0.86 grams

CeF₃ : 1.18 grams

MnCO₃ : 0.80 grams

Na₂ CO₃ : 0.32 grams

EXAMPLE 3 Formulation Sr₃.905 Ce₀.3 Na₀.3 Mn₀.4 (PO₄)₃ F₂

Reagent Blend

CaHPO₄ : 8.16 grams

CaCO₃ : 0.71 grams

CaF₂ : 0.86 grams

CeF₃ : 1.18 grams

MnCO₃ : 0.92 grams

Na₂ CO₃ : 0.32 grams

EXAMPLE 4 Formulation Sr₃.955 Ce₀.3 Na₀.3 Mn₀.35 (PO₄)₃ F₂

Reagent Blend

SrHPO₄ : 8.26 grams

SrCO₃ : 0.90 grams

SrF₂ : 1.04 grams

CeF₃ : 0.89 grams

MnCO₃ : 0.60 grams

Na₂ CO₃ : 0.24 grams

EXAMPLE 5 Formulation Sr₃.905 Ce₀.3 Na₀.3 Mn₀.4 (PO₄)₃ F₂

Reagent Blend

SrHPO₄ : 8.26 grams

SrCO₃ : 0.79 grams

SrF₂ : 1.04 grams

CeF₃ : 0.89 grams

MnCO₃ : 0.69 grams

Na₂ CO₃ : 0.24 grams

EXAMPLE 6 Formulation Ca₃.435 Sr₀.57 Ce₀.3 Na₀.3 Mn₀.3 (PO₄)₃ F₂

Reagent Blend

CaHPO₄ : 6.994 grams

SrHPO₄ : 1.579 grams

CaCO₃ : 0.009 grams

SrCO₃ : 0.002 grams

CeO₂ : 1.033 grams

MnCO₃ : 0.690 grams

Na₂ CO₃ : 0.318 grams

CaF₂ : 1.343 grams

SrF₂ : 0.352 grams

SPECTRAL PROPERTIES OF THE PHOSPHOR Emission Spectra

Ca-fluoroapatites--The corrected emission spectra of FIG. 2 are plottedfor the type 4381 phosphor and for a sample from Example 1 offormulation

    Ca.sub.4.005 Ce.sub.0.3 Na.sub.0.3 Mn.sub.0.3 (PO.sub.4).sub.3 F.sub.2.

The excitation wavelength was 254 nm and the ordinates in FIG. 2 and inthe following figures are proportional to photons/nm. The correctedemission spectra are truncated at ≃615 nm because of instrumentallimitations. Even so, the main features of the phosphor emission areeasily apparent from an inspection of the following figures.

The emission from type 4381 is a composite of Mn²⁺ and Sb³⁺ bands, thelatter extending from ≃400 nm to a peak of ≃500 nm and then merging withthe Mn²⁺ emission with peak at 575 nm. The emission from Example 1consists of a weak band in the long uv with peak at ≃348 nm of peakintensity ≃1/10 that of the yellow emission from Mn²⁺. The emission bandfrom Mn²⁺ peaks at 581 nm and is slightly shifted (5-6 nm) to longerwavelengths with respect to the corresponding band from type 4381. Peakemission intensity is 96% to 97% that of "Yellow Halo" (FIG. 2). Aslightly higher peak (≃97% of type 4381) is shown in FIG. 3 for a samplefrom Example 2 with formulation

    Ca.sub.3.955 Ce.sub.0.3 Na.sub.0.3 Mn.sub.0.35 (PO.sub.4).sub.3 F.sub.2.

A further increase in Mn content very slightly shifts the emission bandto longer wavelengths, but it does not increase the peak intensity. Thisis shown in FIG. 4 for a sample from Example 3 with formulation

    Ca.sub.3.905 Ce.sub.0.3 Na.sub.0.3 Mn.sub.0.4 (PO.sub.4).sub.3 F.sub.2.

The increased Mn content further quenches the Ce emission withoutcontributing to visible emission.

Sr-fluoroapatites--The optimum Mn content for highest peak intensityexceeds that for the case of the Ca fluoroapatites. This is evident frominspection of FIG. 5 where the emission spectra of samples from Examples4 and 5 are plotted. The Ce emission has a doublet structure and isrelatively more intense than the corresponding emission of the Cafluoroapatite analogs. The Mn emission in the Sr fluoroapatites matchesvery closely in band position and width with that of type 4381. Relativepeak height, relative to the former, was 91%, the highest value to date,and 82% for the samples of FIG. 5 with respective formulations

    Sr.sub.3.955 Ce.sub.0.3 Na.sub.0.3 Mn.sub.0.35 (PO.sub.4).sub.3 F.sub.2 (Example 4)

and

    Sr.sub.3.905 Ce.sub.0.3 Na.sub.0.3 Mn.sub.0.4 (PO.sub.4).sub.3 F.sub.2 (Example 5).

Mixed Ca-Sr fluoroapatites--The emission spectra of a mixed Ca-Srfluoroapatite is shown in FIG. 6. The emission band of Example 6 withformulation

    Ca.sub.3.435 Sr.sub.0.57 Ce.sub.0.3 Na.sub.0.3 Mn.sub.0.3 (PO.sub.4).sub.3 F.sub.2

peaks at ≃585 nm and has a height of ≃94% relative to type 4381.Increase in Sr content moves the yellow emission peak further to longerwavelengths.

Excitation Spectra

The presence of Ce³⁺ widens the spectral region that is accessible inthe uv for absorption by the phosphor and subsequent conversion intovisible light. This is shown in FIG. 7. The excitation region of type4381 peaks sharply around 240 nm and then drops rapidly toward longerwavelengths.

Type 4381 therefore absorbs poorly in the long uv and cannot convert thelong wavelength emission components of the plasma. On the contrary, theexcitation bands for the CaFAP phosphors of the present invention covermost of the uv region. This is evident in FIG. 7 for the case of samplefrom Example 1 with formulation

    Ca.sub.4.005 Ce.sub.0.3 Na.sub.0.3 Mn.sub.0.3 (PO.sub.4).sub.3 F.sub.2

and for an analogous Sr fluoroapatite excitation spectrum shown in FIG.8 and referring to Example 4 with formulation

    Sr.sub.3.955 Ce.sub.0.3 Na.sub.0.3 Mn.sub.0.35 (PO.sub.4).sub.3 F.sub.2.

This wider spectral region for uv excitation is a potential advantage ofthe phosphors of the present invention over the commercially used type4381.

Comparison of an Improved Phosphor of the Present Invention with that ofthe Prior Art

The phosphor formulation of Example 6 was compared to that of the priorart teaching given in Example 9 of British Pat. No. 792,589 to Ranby.The respective formulations were:

Example 6 formulation of the present invention:

    Ca.sub.3.435 Sr.sub.0.57 Ce.sub.0.3 Mn.sub.0.3 Na.sub.0.3 (PO.sub.4).sub.3 F.sub.2

Ranby's formulation:

    Ca.sub.3.83 Sr.sub.0.67 Ce.sub.0.19 Mn.sub.0.28 Na.sub.0.52 (PO.sub.4).sub.3 F.sub.1.34 Cl.sub.0.52.

To insure that the comparison was as objective as possible, thefollowing precautions were observed.

(1) Both of the test phosphors were prepared from the same high purityingredients, except NaCl was used for Ranby's formulation, as stated inhis patent, instead of Na₂ CO₃.

(2) The firings were conducted in the same furnace, each under thespecified conditions, using similar quantities of powder blend.

After firing each of the phosphors exhibited in a white body color andemitted in the yellow when excited by short uv radiation. However, acomparison of the emission intensities, as measured by the height of theemission band at 575 nm (254 nm excitation), showed the output of thephosphor made by Ranby's formulation to be only 75% of the output of thephosphor of the present invention.

Additional phosphor examples 7 through 10 of the present invention wereprepared the same way as examples 1 through 6.

EXAMPLE 7 Formulation Ca₃.955 Ce₀.3 Na₀.3 Mn₀.35 (PO₄)₃ F

Reagent Blend

CaHPO₄ : 6.80 grams

CaCO₃ : 1.91 grams

CaF₂ : 0.78 grams

CeO₂ : 1.03 grams

MnCO₃ : 0.80 grams

Na₂ CO₃ : 0.32 grams

(NH₄)₂ HPO₄ : 1.32 grams

EXAMPLE 8 Formulation Ca₃.955 Ce₀.3 Na₀.3 Mn₀.35 (PO₄)₃ F₂

Reagent Blend

CaHPO₄ : 6.80 grams

CaCO₃ : 0.91 grams

CaF₂ : 1.56 grams

CeO₂ : 1.03 grams

MnCO₃ : 0.80 grams

Na₂ CO₃ : 0.32 grams

(NH₄)₂ HPO₄ : 1.32 grams

EXAMPLES 9 Formulation Ca₃.955 Ce₀.3 Na₀.3 Mn₀.35 (PO₄)₃ F₃

Reagent Blend

CaHPO₄ : 6.68 grams

CaF₂ : 2.34 grams

CeO₂ : 1.03 grams

MnCO₃ : 0.80 grams

Na₂ CO₃ : 0.32 grams

(NH₄)₂ HPO₄ : 1.44 grams

EXAMPLE 10 Formulation Ca₃.955 Ce₀.3 Na₀.3 Mn₀.35 (PO₄)₃ F₄

Reagent Blend

CaHPO₄ : 5.32 grams

CaF₂ : 3.12 grams

CeO₂ : 1.03 grams

MnCO₃ : 0.80 grams

Na₂ CO₃ : 0.32 grams

(NH₄)₂ HPO₄ : 2.76 grams

The plaque brightness measurements of examples 7-10, compared to aCaFAP:56, Mn reference phosphor were:

77.5% for Example 7,

92.2% for Example 8,

100.5% for Example 9, and

101.0% for Example 10.

It was observed that the plaque brightness measurements show that theemission intensity increases monotonically with the fluoride content,reaching a maximum in materials formulated with F=4. The phosphorsformulated with the higher fluoride contents (F=3 and 4); however, arenot the preferred phosphors because the fired phosphor cake was veryhard and not easily reduced to the powder form.

The first cooling step where the fired blend is cooled to about 700° C.in about 2 hours prevents green body color regions within the firedphosphor cake.

While there has been shown and described what is at present consideredthe preferred embodiment of the invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the scope of the invention as defined bythe appended claims.

What is claimed is:
 1. A process for producing cerium and manganeseactivated calcium-strontium fluoroapatite luminescent materialcomposition containing sodium, comprising:mixing a calcium source, astrontium source, a cerium source, a sodium source, a manganese source,a phosphate source, and a fluoride source together in amounts inaccordance with the formula

    (Ca.sub.1-s Sr.sub.s).sub.t Ce.sub.v Na.sub.w Mn.sub.x (PO.sub.4).sub.y F.sub.z

whereins is from 0 to 1 t is from about 3.43 to about 4.01 v is about0.3 x is from about 0.3 to about 0.4 y is about 3.00 z is from 1.0 toabout 4.0 to form a reagent mixturewherein said calcium source isselected from the group consisting of calcium hydrogen phosphate,calcium carbonate, calcium fluoride, and mixtures thereof; saidstrontium source is selected from the group consisting of strontiumhydrogen phosphate, strontium carbonate, strontium fluoride, andmixtures thereof; said cerium source is selected from the groupconsisting of cerium (IV) oxide, cerium (III) fluoride, and mixturesthereof; said sodium source consists of sodium carbonate; said manganesesource consists of manganese (II) carbonate; said phosphate source isselected from the group consisting of diammonium hydrogen phosphate,calcium hydrogen phosphate, strontium hydrogen phosphate, and mixturesthereof; and said fluoride source is selected from the group consistingof calcium fluoride, strontium fluoride, cerium (III) fluoride, andmixtures thereof; comminuting said reagent mixture to form a thoroughlymixed powder blend, firing said blend for about 1 to about 1.5 hours atabout 900° C. to about 1050° C. in about 1.0 l/minute flow of N₂containing 5% H₂ and a small amount of water vapor; cooling the firedblend to about 700° C. in about 2 hours; further cooling the fired blendto about 25° C.; and milling the cooled blend.
 2. A process according toclaim 1 wherein the N₂ containing 5% H₂ contains water vaporincorporated therein by bubbling it through water maintained at atemperature of 21° C.